Solutions of electrolytes in organic solvents such as propylene carbonate, γ-butyrolactone, and acetonitrile have been used as nonaqueous electrolytes for electrochemical devices such as lithium batteries, electric double-layer capacitors, electrochromic devices, and dye-sensitized solar cells. However, organic solvents used for these electrolyte solutions volatilize easily and besides they themselves are hazardous materials; therefore, concern has been expressed over their reliability, durability, and safety over a long period of time.
One of the methods to solve the aforementioned problems is to use an ionic liquid for an electrolyte. The term “ionic liquid” is commonly used for a compound which consists of a combination of anions and cations and has a melting point of 100° C. or below and it is reported in the non-patent document 1 that a combination of ions to suit the purpose can display required characteristics.
Several proposals have been made on electrolytes useful for electrochemical devices. For example, alkyl-substituted imidazolium salts are described in the patent documents 1-4 and quaternary alkylammonium salts in the patent documents 5-8.
In spite of the aforementioned report on the possibility that a combination of ions to suit the purpose can display required characteristics, what are actually used in most of current developmental works are quaternary imidazolium salts, alicyclic quaternary ammonium salts, quaternary alkylammonium salts, and the like and it has been considered necessary to develop materials of a novel skeleton for a breakthrough.
The inventors of this invention have searched for a novel skeleton aiming at developing electrolytes useful for electrochemical devices and devoted attention to a singularity in the chemical structure of guanidine compounds, namely, the structure of nonlocalized electrons. Guanidine compounds have hitherto been used as raw materials for pharmaceuticals, dyes, paints, photographic chemicals, and additives for polymers. Regarding the electrochemical properties of guanidine compounds, reports on the Bronsted acid salts of acyclic tetramethylguanidine are known (patent document 9 and non-patent document 2), but the salts are limited to protonic compounds. Regarding quaternary guanidine compounds, a compound having a hexaalkylguanidine cation is reported (non-patent document 2), but an account in the report is limited to applications of acyclic quaternary guanidine compounds to dye-sensitized solar cells.
Patent document 1: JP8-259543 A
Patent document 2: JP2003-62467 A
Patent document 3: JPH11-86905 A
Patent document 4: JP347213 C
Patent document 5: WO02/076924 A
Patent document 6: JP2003-331918 A
Patent document 7: JP2981545 C
Patent document 8: JP2004-67543 A
Patent document 9: JP2004-253357 A
Non-patent document 1: Chem. & Eng. News, May 15, 2000
Non-patent document 2: Synthetic Communication, Vol. 34, pp. 3083-3089 (2004)
Non-patent document 3: Appl. Phys. A, 79, 73-77 (2004)